The present invention relates to optical computing devices and, more particularly, to optical design techniques for generating environmentally resilient optical elements used in optical computing devices.
Optical computing devices, also commonly referred to as “opticoanalytical devices,” can be used to analyze and monitor a substance in real time. Such optical computing devices will often employ a processing element that optically interacts with the substance to determine quantitative and/or qualitative values of one or more physical or chemical properties of the substance. The processing element may be, for example, an integrated computational element (ICE), also known as a multivariate optical element (MOE), which is essentially an optical interference filter that can be designed to operate over a continuum of wavelengths in the electromagnetic spectrum from the UV to mid-infrared (MIR) ranges, or any sub-set of that region. Electromagnetic radiation that optically interacts with the ICE is changed so as to be readable by a detector, such that an output of the detector can be correlated to the physical or chemical property of the substance being analyzed.
An ICE typically includes a plurality of optical layers consisting of various materials whose index of refraction and size (e.g., thickness) may vary between each layer. An ICE design refers to the number and thickness of the respective layers of the ICE component. The layers may be strategically deposited and sized so as to selectively pass predetermined fractions of electromagnetic radiation at different wavelengths configured to substantially mimic a regression vector corresponding to a particular physical or chemical property of interest. Accordingly, an ICE design will exhibit a transmission function that is weighted with respect to wavelength. As a result, the output light intensity from the ICE conveyed to the detector may be related to the physical or chemical property of interest for the substance.
It has been found, however, that the resulting transmission function for some ICE designs may change or shift over a range of environmental conditions. For example, an ICE employed in a downhole environment, such as is common in the oil and gas industry, might be expected to operate in temperatures ranging between 150° F. and 350° F., in pressures ranging between 3,000 psi and 20,000 psi, and at an absolute humidity reaching 15%. In such extreme environmental conditions, it is preferred that ICE components maintain a standard prediction error of less than 2% over the range of concentration of an analyte under study. Due to temperature fluctuations, however, the material refractive indices and layer thicknesses for some ICE designs may fluctuate, thereby adversely affecting the corresponding transmission function. In other cases, the effects of humidity (related to temperature and pressure) may cause a spectral shift due to interaction with surface material, thereby providing faulty or otherwise inaccurate predictions.
What is needed, therefore, are systems and methods of designing and optimizing ICE components that are robust with regards to spectral error arising from calibration errors and environmental factors that have a critical impact in, for example, oilfield applications.